Heat cooking oil in a skillet. Cook the onion and garlic until tender.Add the pork and cook until it is heated through.Pour into a bowl.Heat the tortillas just until pliable.Divide the pork onto the center of each of the tortillas.Top with the cheese, salsa, and black olives.Fold the tortilla over.Melt the butter in the skillet.Grill the tacos on each side for 2 to 3 minutes or until the cheese is melted.